1. Field of the Invention
The present invention relates generally to battery-operated wireless communication devices, such as radios, and in particular, to estimating the useful power remaining available in the batteries of such radios.
2. Background of the Invention
Batteries are utilized to power portable radios, such as two-way radios, cell phones, pagers, and the like. It is often necessary to estimate battery voltage during operation of the radio to ensure the radio has sufficient operating power and, more particularly, voltage for proper operation. In some radios, these estimates result in battery voltage levels, which are in turn translated into discrete sets of battery fuel levels which can be associated with fuel xe2x80x9cbarsxe2x80x9d that are displayed on a visual display of the radio.
It is well-known in the art that the voltage of a battery xe2x80x9cslumpsxe2x80x9d according to the load placed on it. Radios are typically not configured to detect a minimum transmit battery voltage (Vtx min) that corresponds to a maximum or peak current draw during in-slot transmission. However, radios are often able to detect an average in-slot transmit battery voltage (Vin-slot) and an average out-of-slot transmit battery voltage (Vout-slot). As used herein, Vin-slot and Vout-slot refer to slots or windows of time when the radio transmits data and stays the transmission of data during a transmission mode of operation, respectively. In prior art radios, Vtx min is estimated by multiplying the difference between Vin-slot and Vout-slot by a constant compensation factor (K). The constant compensation factor K is determined empirically from bench measurement and then programmed as a fixed variable in the radio. A typical formula for use in calculating battery voltage levels in both stand-by and transmission modes is represented as follows:
Vtx min=(Vout-slot)xe2x88x92[(Vout-slot)xe2x88x92(Vin-slot)]*K
where Vout-slot is assumed to equal a stand-by battery voltage (Vstd-by) and Vtx min equals the minimum transmit battery voltage.
Vtx min is then compared against a set of predetermined loaded voltage thresholds to determine the fuel xe2x80x9cbarsxe2x80x9d to be displayed on the visual display of the radio and to estimate the useful power remaining in the battery.
There are two basic problems with the above-described approach. First, Vout-slot is assumed to equal Vstd-by. This assumption, however, is incorrect since the radio continues to draw power during the out-of-slot transmission mode that it would not draw when in stand-by mode. Specifically, the voltage of the battery while the radio is in stand-by mode will be greater than the voltage of the battery while the radio is in out-of-slot transmission mode. This difference causes the voltage threshold comparison for stand-by mode to need to be adjusted accordingly.
The second shortcoming with the prior art approach arises when Vtx min is compared to the set of predetermined voltage thresholds which are determined based on the assumption that the radio is operating at substantially full transmission power. However, the more aggressively the radio reduces its transmission power, the more this comparison will be adversely affected. This is especially true when the radio is operating in a GSM mode of operation where the radio more aggressively reduces its transmission power output as a function of the strength of the signal received by the radio.